This invention relates to the field of liquid crystal polymer processing. The invention further relates to a method for making stock shapes from a composition which includes an aromatic polyester sold under the trademark "Xydar.RTM.".
Low molecular weight liquid crystalline compounds have been known for many years. Liquid crystal polymers (LCP's) have attained greater prominence in only the last 20 years, however, due to the discovery of higher molecular weight aramids and thermotropic polyesters. LCP's are generally divided into two groups depending upon whether they exhibit liquid crystalline or anisotropic order in solution (lyotropic) or in the melt phase (thermotropic). The presence of such crystals (or crystalline properties) in a particular polymer may be confirmed by conventional polarized light techniques using crossed polarizers.
Thermotropic liquid crystal polymers have been described in numerous ways by such terms as "liquid crystalline", "liquid crystal" or "anisotropic". It is believed that the polymers comprising this group involve a parallel ordering of molecular chains. Depending upon the order of molecules relative to one another, liquid crystal polymers may be characterized as possessing a mesophasic structure which is either: cholesteric, smectic or nematic. Thermotropic LCP's include, but are not limited to, wholly aromatic polyesters, aromatic-aliphatic polyesters, aromatic polyazomethines, aromatic polyester-carbonates and wholly (or non-wholly) aromatic polyester-amides. Typically, LCP's are prepared from long and flat monomers which are fairly rigid along their molecular axes. These polymers also tend to have coaxial or parallel chain-extending linkages therebetween. To be considered wholly aromatic, each monomer of an LCP must contribute at least one aromatic ring to the polymeric backbone. It is believed that such universal contribution of aromatic rings enables these polymers to exhibit anisotropic properties in their melt phases.
Some of the monomers used for forming thermotropic LCP's include those derived from aromatic diols, amines, diacids and/or aromatic hydroxyacids. For some time now, there has been produced a family of polymers notable for their high temperature performance and self-reinforcement properties. The aromatic polyester sold by Dartco Manufacturing, Inc. of Augusta, Ga. under the trademark "Xydar.RTM." consists essentially of p, p' biphenol, p-hydroxybenzoic acid, and terephthalic acid monomers. Xydar.RTM. is commercially available in various grades including a substantially pure (or neat) form, Xydar.RTM. SRT-300, and the lower grade SRT-500 product. Xydar.RTM. powders are also combinable with certain fillers. For example, Xydar.RTM. MD-5 is approximately 50% glass-filled, FSR-315 is about 50% talc-filled, and FC-130 contains about 50% mineral filler.
Xydar.RTM. is an attractive thermotropic LCP for various applications due to the following properties. In its neat form, SRT-300 possesses a deflection temperature of about 671.degree. F. under a flexural load of about 264 psi. Xydar.RTM. SRT-300 can be used in continuous electrical service at temperatures as high as 464.degree. F. for over 100,000 hours. When subjected to a three-point bend test, injection molded articles made from this LCP material possess an overall toughness of about 4.1 ksi-in.sup.1/2. The material is essentially inflammable and radiation resistant. It generates very little smoke and does not drip when exposed to live flame. Xydar.RTM. also serves as an excellent electrical insulator with high dielectric strength and outstanding arc resistance. It resists chemical attack from most polar and nonpolar solvents, including but not limited to: hot water, acetic acid, other acids, methyl ethyl ketone, isopropyl alcohol, trichloroethylene, caustics, bleaches and detergents. Xydar.RTM. resins are virtually uneffected by exposure to hydrocarbons and have very low coefficients of friction. This material also possesses an ability to retain substantially high strength levels at relatively high temperatures. For example, the tensile strength and tensile modulus of an injection molded SRT-300 article change with temperature as follows:
TABLE I ______________________________________ Tensile Strength Tensile Modulus Temperature (KSI) (MPSI) ______________________________________ 73.degree. F. 20 2.5 450.degree. F. 6.4 1.5 575.degree. F. 3.8 1.2 ______________________________________
The effective crystallization temperature of Xydar.RTM. SRT-300 is about 702.degree. F. By this term, it is meant that at temperatures below about 702.degree. F., Xydar.RTM. exhibits nematic behavior. At or above this temperature, the liquid crystalline state allows Xydar.RTM. to flow more readily. Substantially pure Xydar.RTM. possesses a melting point of about 790.degree. F., above which the material begins to degrade. Xydar.RTM. also exhibits exceptional resistance to thermal oxidative degradation. Its decomposition temperature, as determined by thermogravimetric analysis, is about 1040.degree. F. when measured in air and about 1053.degree. F. in a nitrogen atmosphere.
With such excellent strength, lubricity, chemical resistance and other properties for temperatures ranging from below zero to its melting point of 790.degree. F., Xydar.RTM. should be useful for a wide range of applications. Because of its ability to withstand exposure to aeronautic hydraulic fluid, jet fuel, leaded gasoline, brake and transmission fluids, and ethylene glycol coolants, Xydar.RTM.-containing compositions could be used to make internal components, fuel system parts, engine bearings, and other brackets, fasteners or housings for the automotive and/or aerospace industries. For the electronics industry, Xydar.RTM.-containing sockets, chip carriers, high temperature connectors, and/or switches are envisioned. For the field of fiber optics, connectors, couplers, and buffers may be constructed from this material. Xydar.RTM. might also find its way into such other consumer goods as household appliances, microwaveable housewares, and business, sports or recreation equipment.
Despite the foregoing list of potential applications, current Xydar.RTM. usage remains limited by processing constraints. Prior to this invention, it was known to screw extrude or otherwise compact Xydar.RTM. powders into pellets (or other particles of a sufficient size) for subsequent injection molding at temperatures substantially near or above its crystallization temperature. In promotional literature, Dartco recommends processing pelletized Xydar.RTM. after the material has been dried at 300.degree. F. for about eight hours to remove any surface moisture. These pellets may then be heated between about 752.degree.-806.degree. F. within a vented screw extruder having an injection pressure of about 14 ksi and a 20-24:1 length to diameter (L/D) ratio. Dartco further recommends using compression ratios between 2 or 3:1. Preferably, the mold into which Xydar.RTM. is injected should also be heated to at least about 464.degree. F. for promoting better, consistent material flow throughout.
Injection molding of Xydar.RTM. into consumable goods, or parts for the same, is not without its limitations. Constraints relating to how rapidly Xydar.RTM. cools and/or hardens limit the size of molds into which Xydar.RTM. may be injected. These molds may have some disparity in width and/or length depending on the number of areas from which Xydar.RTM. is injected. The maximum cross-sectional thickness of Xydar.RTM. injection molds rarely, if ever, exceeds about 0.25 inch, however. Although occasional attempts at injecting Xydar.RTM. into slightly thicker molds may succeed, Xydar.RTM. cannot consistently fill such molds on a complete basis, thus resulting in products which have undesirable voids of material. Because of similar processing constraints, it is not possible to consistently or commercially injection mold Xydar.RTM. into products weighing about four or more pounds. When injection molding Xydar.RTM. in such quantities, incomplete mold filling more commonly results.
Other proposed techniques for processing Xydar.RTM. have been unsuccessful. Several attempts were made at extruding Xydar.RTM. in the solid state using the process parameters typically associated with extruding other polymers. These attempts only resulted in the production of defective stock shapes having repeated divergings into adjacent subsections. It is believed that such defects were due to die plugging. The end result was non-machinable and contained multiple fractures.